Polymer compositions which contain active compounds such as insecticides in polymers, and molded articles shaped of the same polymer compositions hitherto have been known and have been used in a wide rage of fields, since molded articles in various shapes can be obtained at relatively low costs. These molded articles exhibit their actions by allowing the active compounds to transpire, or by oozing the active compounds to the surfaces of the molded articles to thereby release the active compounds, i.e., so-called bleeding (cf. Patent Publication 1).
When an active compound having a low transpiring property is used, such a compound hardly releases from a molded article by its transpiration, and thus releases therefrom mainly by bleeding. Bleeding arises when the active compound in an amount exceeding a saturated amount to the polymer (an oversaturated amount) is retained in the molded article, and this is a phenomenon that the oversaturated amount of the active compound (=the amount of the active compound added−the saturated amount of the active compound in the molded article) migrates to the surface of the molded article with time. When a molded article is shaped of a polymer composition which contains an active compound in an amount exceeding a saturated amount, the active compound bleeds out to the surface of the molded article with time. However, it is generally known that this bleeding rate varies significantly depending on the amount of the active compound initially added, and this bleeding rate tends to decrease with time. On the other hand, when an molded article exhibits its action by way of bleeding of an active compound, a period of time during which a desired bleeding rate is obtained becomes an index for an effective period of the molded article. Therefore, once an effective period of the molded article has been determined, the amount of the releasable active compound initially added is inevitably determined.
To obtain a molded article usable over a long period of time, it is needed to use a polymer composition which contains an active compound in an amount exceeding a saturated amount to a polymer. However, a molded article shaped of a polymer composition which contains an oversaturated amount of an active compound in a polymer permits a large amount of the active compound to bleed out at an early stage of use thereof, and thus, it is impossible to maintain a bleeding rate during a desired period of time.
Patent Publication 1: JP-A-6-315332